Malignantly transformed cells secrete a variety of proteases and protease precursors which have been hypothesized to be involved in the invasiveness, metastasis, immune suppression, and paraneoplastic syndromes associated with cancer. Our work has concentrated on the function of a secreted lysosomal cysteine proprotease called procathepsin L or MEP (for major excreted protein of transformed cells). Cathepsin L is a broad spectrum acid protease found both in lysosomes and secreted as the proenzyme into the extracellular space. We have cloned the genes for mouse and human procathepsin L and compared their structures and modes of regulation. Each has 8 exons and 7 introns located at the same positions within the coding region, but the structural gene for procathepsin L in the mouse spans 7400 bp, while the human procathepsin L gene is 5100 bp, with the size difference accounted for almost entirely by intron size. Unlike the mouse gene which has a unique start site of transcription and a single major mRNA species, at least two major mRNAs are transcribed from the human procathepsin L gene. These two mRNAs differ in their 5' untranslated sequences, and represent alternative splicing or alternative promoters. To determine why procathepsin L is secreted in large amounts by the activity of malignantly transformed cells and other cells which overproduce it, we have initiated a deletion analysis of the human procathepsin L cDNA. Expression of deleted forms of human procathepsin L in mouse NIH 3T3 cells results in either failure to form a stable structure (deletions of the "pro" piece) or failure to be secreted (carboxy-terminal deletions). To evaluate the function of secreted procathepsins in metastasis we have compared the synthesis, secretion, and activity of cathepsins L and B in a series of mouse B 16 melanoma cells of varying metastatic potential, and find no clear correlation with ability to metastasize.